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Beomaster 8000 restoration #3

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ALF
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ALF posted on Tue, Jun 4 2019 9:38 AM

Hi All,

its sometimes really good to have a donor unit at hand.....

as today, the fm-signal on the recently restored BM8000 sounded distorted, no matter which station

and the volume of that signal was quite loud even at vol setting 1 ?!

after checking that other inputs are ok I decided to swapp over the fm-related boards #2 & 1 from the donor unit and everything sounded fine again....😁🙏

but,.....

where to look for the most likely problem source on those two boards ??

as I have decided to go for another restoration project I want those fm- boards to work again.

the tuning voltage was set correctly.....other trim pots on the tuner boards I won’t dare to touch đŸ˜ŗ

the donor unit is in far too very nice condition - so, its worth it to put in the work.

could someone throw some pointers in my direction.....pleeeease 🙄

ALF

 

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ALF
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ALF replied on Tue, Jun 4 2019 10:27 AM

Just had another look at the SM circuit diagram 1:

would it be a good start checking that OP amp 2IC6 ??

that one is handing over the FM signal  via P10/pin1 and 4 to the preamp - board #3 P19

ALF

Lonnie
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Hi ALF,

 

Is the distortion in one channel or both?

2IC6 is a good place to start. What are the DC voltages on the chip?

I had a TL072 fail on the filter board 4IC2 on a BM8000 for no apparent reason, it was distorting badly. It failed after I'd changed all the caps, I replaced it with no other changes and it was all good.

 

Regards

 

Lonnie

ALF
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ALF replied on Sat, Jun 8 2019 3:24 PM

Hi Lonnie,

the distortion was in both channels....all fine after I swapped boards.

in the meantime I replaced 2IC6 but won’t be able to test until all other boards are done.

let’s hope it did the trick ?!!

only the processor board to do (i hope) and thinking about changing the two crystals as well while I am at it ??

they should both be short-wired before being put in, right ?

will keep all posted

ALF

sonavor
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Hi Alf,

I wouldn't change the microcomputer crystal oscillators unless you know they are faulty or doing a restoration where the owner lives far away and shipping costs are outrageous.

 

The microcomputer board is easy enough to get to and can be done later if necessary. Why risk it when you don't have to?

 

If you end up having to I believe I sent you links before to the details Rudy and I have posted on the Beolover Blog.  You can't just change only the crystal oscillators either. Each oscillator has two capacitors that control the oscillator frequency.

 

John (sonavor)

ALF
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ALF replied on Sun, Jun 9 2019 3:22 AM

Hi John,

as always, good to hear from you 😁 thank you for your advice - point taken !

going through Rudys blogs though it just struck me as ‘one of the things to do’ when a BM8000 restoration is under way?!

down here it is not as easy to get access to those parts, so I grab it while I can - yes, the two crystals with two pairs of 18nF caps 🤙😁

will store them just in case.

the processor board needs attention:

the memory chip seems to no longer does its job as FM stations won’t get stored,

the 22uF cap needs replacing and I was going to

provide the two big processors decent tulip-style sockets........

that is it for now - looking forward to read further comments, ideas etc.

ALF

sonavor
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As I said, if the BM8000 is being restored for an owner that lives somewhere far from us, where shipping cost is a factor (i.e. USA to Australia), then we change out the oscillator devices as extra insurance. 

John

ALF
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ALF replied on Mon, Jun 10 2019 5:44 AM

Bad news I am afraid:

fuses blowing on the #7 board and I fear the standby transformer has a short ?!

if 3OT1 is disconnected from #7 board fuses stay on, if reconnected fuses blow !

it is not the power supply board as after disconnecting P52 fuses still blowing đŸ¤ŦđŸ˜ĸ

looks like I have to chase the almost impossible replacement for that standby transformer đŸ˜ĸ

unless one fellow Beoworld member can help ?

ALF

sonavor
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That smaller transformer is for the 5 VDC and +-15VDC.

I am not presently in a location I can check anything so I'm just going from memory. There are other things besides the small transformer that could cause your problem.  You could always pull the small transformer and check it out by itself.

ALF
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ALF replied on Mon, Jun 10 2019 10:15 AM

Hi John,

yes, understood , it is for those 3 voltages.

the transformer is disconnected from the PS board #6 !!

the moment the HV side of the transformer is connected to board #7 the fuse blows 

if the HV side is dis-connected the fuses stay on. The HV is getting about 238VAC, coming straight from the AC mains.

I believe board #7 is doing its job.

perhaps there are other sources causing that problem - sadly I do not have the equipment to do any further testing of that stand-by transformer.

are you be any chance implicating the main transformer ?

ALF

sonavor
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sonavor replied on Mon, Jun 10 2019 11:26 AM

No, both transformer s are "main" transformers. The large transformer is for the amplifier +-55V rails. The smaller transformer is for everything else. It sounds like something that gets its power through +5V or +-15V is shorting and blowing the fuses.

You could try disconnecting the power cables from some of the other boards to eliminate them as the source of the problem. For example the FM boards then the preamp board.

ALF
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ALF replied on Mon, Jun 10 2019 11:54 AM

I am trying to follow John, but

if P52 is disconnected from the PS board #6, which is fed by the small transformer, how do you get power

through the +5V/+/-15V regulators to other boards ??

plus, if the HV side is connected to the small transformer and P52 is not connected to board#6 how can this blow the fuse unless

the small transformer itself has a short ?? i am really trying to understand and shall have another look !

ALF

ALF
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ALF replied on Mon, Jun 10 2019 1:25 PM

Sorry to say John, but

after disconnecting all power sources from board #6 the fuse still blows the moment I reconnect the HV side of the small transformer !

I am reasonably sure that transformer has a short ☚ī¸

I would not hesitate buying one if I can find one.......do you have a spare to sell ??

ALF

Dillen
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Dillen replied on Mon, Jun 10 2019 2:02 PM

Is that part# 8013222?

Martin

ALF
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ALF replied on Tue, Jun 11 2019 9:25 AM

No Martin, it isn’t đŸ¤Ŧ it is 8013227

All I can say is : TYPE 1903 !!!

a brief explanation:

with no additional information from the German seller the unit has been bought from Germany, earmarked as a donor unit originally.

i did not even assume for a moment that this would be a US model, running on 120VAC as there was no US plug attached.

Thanks again Martin for being the catalyst in asking for the part number, only then it triggered me to have another look in the SM ! đŸ˜ŗđŸ˜ŗđŸ˜ŗđŸ˜ŗ

silly mistakes do happen.

all fuses are now staying healthy.

I am however happily reunited with the old issue of ‘no audible sound’.

after adjusting the ‘no load current’ and DC offset there was hope, but....nothing so far.

one usually hears a relay clicking when the power plug is pulled but that is not always the case.....meaning when I reconnect the unit the output 

seems full on without actually using the keyboard or remote. They other issue is the failing ‘store’ function despite having replaced the associated IC

on the uP board!

so, there are still a couple of issues to follow up.

i would most certainly appreciate all the help I can get 🙄🤗🙏

ALF

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